1. Field of the Invention
This invention relates to a hydrogen-absorbing alloy electrode capable of reversibly carrying out the electrochemical charge/discharge of a hydrogen gas which is used as a negative electrode of an nickel-metal hydride battery, and more particularly to surface-treatment of the hydrogen-absorbing alloy particle containing a rare-earth element and nickel.
2. Description of the Related Art
In recent years, in order to realize the alkaline storage battery with high energy density, the nickel-metal hydride battery equipped with a hydrogen-absorbing alloy electrode has been put into practice. Previously known hydrogen-absorbing alloys which can be used for the nickel-metal hydride battery are Tixe2x80x94Ni alloy, La (or Mm)xe2x80x94Ni alloy, etc.
The hydrogen-absorbing alloy used in the hydrogen-absorbing alloy electrode is ground alloy which is obtained by grinding an alloy ingot, flake or spherical particle mechanically and electrochemically, or spherical or similar shape (elliptical) particle which is made by an atomization, a rotary disk method, a rotary nozzle method, etc. Meanwhile, the surface of the hydrogen-absorbing alloy particle is very active. Therefore, when it is exposed to air atmosphere just a little, it immediately reacts with oxygen in the air. As a result, the alloy surface is oxidized to form an oxide layer. The oxide layer attenuates the surface activity of the alloy, and particularly gives rise to reduction of the initial discharging capacity of the battery. Therefore, after the battery has been assembled, it must be subjected to charging/discharging several cycles to several tens cycles to remove the oxide layer and activate the particle surface, thereby realizing the desired discharging capacity. This required very great labor and time.
In order to obviate such inconvenience, a technique for removing an oxide layer on the surface of the hydrogen-absorbing alloy particle has been proposed in JP-A-5-225975 which discloses an acidic treating method of immersing the hydrogen-absorbing alloy particle in hydrochloric acid (HCl).
The acidic treating method proposed by JP-A-5-225975 was efficient to remove the layer of the rare-earth metal oxide of the oxide layer on the surface of the hydrogen-absorbing alloy particle, but not so efficient to remove nickel oxide or nickel hydroxide, thus presenting a problem that the nickel hydroxide is newly created.
Further, by the acidic treatment using hydrochloric acid, chloride ions were left on the surface of the hydrogen-absorbing alloy particle so that rinsing treatment must be made to remove the chloride ions after the acidic treatment. During the rinsing treatment, an inconvenience that the activated surface of the hydrogen-absorbing alloy is oxidized again by oxygen in water was presented.
In order to obviate such inconvenience, a surface treatment of a hydrogen absorbing alloy particle in an atmosphere of a hydrogen gas maintained at high temperature was proposed by JP-A-9-237628 in which before preparing a hydrogen-absorbing alloy electrode, the hydrogen-absorbing alloy particle containing nickel with a nickel oxide or nickel hydroxide is held for a suitable time in an atmosphere of a hydrogen gas (also referred to as hydrogen gas atmosphere) maintained at a temperature where absorbing of the hydrogen gas does not substantially occur so that the layer on the hydrogen-absorbing alloy particle is reduced by the hydrogen gas to expose the nickel activated surface onto the surface of the hydrogen-absorbing alloy particle.
In the hydrogen gas reduction treatment technique for the nickel-containing hydrogen-absorbing alloy proposed by JP-A-9-237628, before the hydrogen gas reduction treatment, the alloy is previously subjected to alkaline treatment to remove the oxide on the surface of the hydrogen-absorbing alloy particle, or previously subjected to acidic treatment to generate a layer of nickel oxide or nickel hydroxide so that the quantity of nickel to be reduced by the hydrogen gas reduction treatment is increased.
However, since the acidic treatment or alkaline treatment which is carried out before the hydrogen reduction treatment belong to a wet type, the nickel-containing hydrogen-absorbing alloy particle must be dried before it is subjected to the hydrogen gas reduction treatment. This presented a problem of making the manufacturing process complicate and increasing the treatment time.
When the nickel-containing hydrogen-absorbing alloy particle is dried before it is subjected to the hydrogen gas reduction treatment, a problem occurred that a part of the surface of the hydrogen-absorbing alloy particle is oxidized during drying so that rare-earth oxide or nickel oxide is formed. In this case, another problem occurred that the nickel oxide of the oxides thus formed can be reduced by the hydrogen gas reduction treatment, but the rare-earth oxide cannot be reduced so that the porosity of the surface of the hydrogen-absorbing alloy particle is decreased, thus attenuating the battery capacity.
Still another problem occurred that where the activated surface of nickel is exposed to the particle surface of the hydrogen-absorbing alloy particle, if the activated surface is exposed to air again, it is oxidized again so that it is difficult to maintain the activity.
A further problem occurred that the since the surface of the hydrogen-absorbing alloy particle is activated greatly by the hydrogen gas reduction treatment, the hydrogen-absorbing alloy particle thus treated must be held in water so that the active surface is lost.
This invention has been accomplished to solve the problems described above, and aims at providing a treatment method which can maintain the activity of the surface of a hydrogen-absorbing alloy particle is activated by hydrogen gas reduction treatment, thereby obtaining a hydrogen-absorbing alloy electrode with an excellent discharging capacity.
In order to attain the above object, the method of manufacturing a hydrogen-absorbing alloy electrode according to this invention comprises:
a step of holding a hydrogen-absorbing alloy particle in an atmosphere of a hydrogen gas maintained at a temperature where absorbing of the hydrogen gas does not substantially occur;
a step of naturally cooling the atmosphere from the temperature where absorbing of the hydrogen gas does not substantially occur to the temperature where the equilibrium hydrogen pressure of the hydrogen-absorbing alloy is equal to the hydrogen pressure in the atmosphere of the hydrogen gas and thereafter vacuum-evacuating the atmosphere of the hydrogen gas so that the hydrogen-absorbing alloy particle is cooled to room temperature while the hydrogen gas is exhausted;
a step of exhausting the hydrogen gas and cooling the atmosphere to room temperature and thereafter introducing at least one kind of gas selected from the group consisting of argon, nitrogen and carbon dioxide into the atmosphere, thereby returning the atmosphere to normal atmospheric pressure; and
a step of immersing the hydrogen-absorbing alloy particle thus acquired in solution containing an oxidation inhibiting agent.
Preferably, said step of holding comprises a step of reducing an oxide or hydroxide residing on the surface of a hydrogen-absorbing alloy particle while the alloy particle is held in an atmosphere of a hydrogen gas maintained at a temperature where absorbing of the hydrogen gas does not substantially occur.
In the case thwhereat the hydrogen-absorbing alloy is pressurized by hydrogen at a prescribed temperature, a metallic phase with the hydrogen solid-solved (xcex1 phase) is first created. When the pressure is further raised, the hydrogen generates a hydroxide (xcex2 phase) where the hydrogen occupies the site of a metallic lattice. At this time, there is a region where the a phase and the xcex2 phase exist commonly, which is referred to as a plateau region. At a high temperature, the pressure of generating the plateau region (plateau pressure) is raised so that the hydroxide cannot be generated. In this state also, it should be noted that the solid-solved state of the hydrogen has occurred so that a little hydrogen exists in the xcex1 phase.
Therefore, no occurrence of the substantial absorbing of hydrogen means the absorbing of only the hydrogen in the solid-solved state in the a phase. In this state, the amount of the occluded hydrogen is much less than that in the region of creating the hydroxide (co-existing region of the xcex1 phase and the xcex2 phase) so that the hydrogen is not substantially charged. If the alloy-absorbing alloy is held in an atmosphere of the hydrogen gas which is lower than the temperature where the hydrogen gas is not substantially absorbed, the hydrogen gas does not contribute to reduction of the oxide or hydroxide on the surface of the alloy particle, but is internally absorbed in the hydrogen alloy particle from its surface.
When the temperature of the atmosphere of the hydrogen gas rises, the equilibrium hydrogen pressure of the hydrogen-absorbing alloy also rises so that hydrogen absorbing does not occur. The hydrogen gas in the atmosphere acts on the layer of the oxide residing in the particle surface so that the oxide on the surface of the hydrogen-absorbing alloy particle is reduced into the metallic state.
In this way, the hydrogen gas reduction to the hydrogen-absorbing alloy particle can reduce the layer of the nickel oxide, nickel hydroxide, cobalt oxide or cobalt hydroxide formed on the surface of the hydrogen-absorbing alloy particle into its metallic state of the nickel or cobalt. This enhances the surface activity of the hydrogen-absorbing alloy particle. Therefore, the hydrogen-absorbing alloy electrode made from the hydrogen-absorbing alloy particle subjected to the above reduction is excellent in the activity and high in the initial discharging capacity. Accordingly, the number of times of the charging/discharging carried out for the battery incorporating the hydrogen-absorbing alloy electrode can be reduced.
However, when the hydrogen-absorbing alloy particle with the surface activity improved is exposed to the air, it is oxidized again so that its activity is difficult to maintain. In order to obviate such inconvenience, in accordance with this invention, the surface of the hydrogen-absorbing alloy particle is caused to adsorb the gas such as argon, nitrogen or carbon dioxide, or otherwise the hydrogen-absorbing alloy particle is immersed in the solution containing the oxidation inhibiting agent so that the surface of the hydrogen-absorbing alloy particle is not directly exposed to the air. Thus, even when the surface of the hydrogen-absorbing alloy particle is exposed to the air, it is prevented from being oxidized again, thereby maintaining the surface activity.
Meanwhile, in order to introduce the gas such as argon, nitrogen or carbon dioxide into the atmosphere, the atmosphere of the hydrogen gas must be vacuum-evacuated to exhaust the hydrogen gas. However, if the temperature of the atmosphere is lowered during exhaustion of the hydrogen gas, the hydrogen-absorbing alloy will adsorb the hydrogen gas. In order to obviate such inconvenience, in accordance with this invention, the atmosphere is naturally cooled from the temperature where absorbing of the hydrogen gas does not substantially occur to the temperature where the equilibrium hydrogen pressure of the hydrogen-absorbing alloy is equal to the hydrogen pressure in the atmosphere of the hydrogen gas and thereafter vacuum-evacuation of the atmosphere of the hydrogen gas is started so that the atmosphere is cooled to room temperature while the hydrogen gas is exhausted.
When the equilibrium hydrogen pressure of the hydrogen-absorbing alloy becomes equal to the hydrogen pressure in the atmosphere of the hydrogen gas, the hydrogen-absorbing alloy starts to charge the hydrogen. If the vacuum-evacuation is started at this time to exhaust the hydrogen gas, since the hydrogen pressure in the hydrogen gas atmosphere falls, the charged hydrogen will be desorbed. Thus, the absorbing/desorbing of hydrogen is made the moment the vacuum-evacuation is started. As a result, minute-cracks are formed on the surface of the hydrogen-absorbing alloy particle so that the surface area of the hydrogen-absorbing alloy particle is also increased.
Thereafter, even if the atmosphere is vacuum-evacuated so that its temperature is cooled to room temperature, the hydrogen does not remain in the atmosphere so that the hydrogen-absorbing alloy particle does not charge the hydrogen gas. When the gas such as argon, nitrogen or carbon dioxide is introduced in the atmosphere at a normal temperature (25xc2x0 C.), the gas such as argon, nitrogen or carbon dioxide is adsorbed on the surface of the hydrogen-absorbing alloy particle.
Since the gas such as argon, nitrogen, carbon dioxide adsorbed on the hydrogen-absorbing alloy particle is removed by the alkaline electrolyte in the battery, it does not exert an adverse effect on the battery. Since the active surface is exposed within the battery, the initial discharging capacity of the battery is improved greatly. Further, when the hydrogen-absorbing alloy particle with the gas such as argon, nitrogen, carbon dioxide, etc. is immersed in the solution containing the oxidation inhibiting agent, the oxidation inhibiting agent is selectively adsorbed on the surface of the hydrogen-absorbing alloy particle so that the creation of the oxide or hydroxide is inhibited, thereby maintaining the surface activity.
In this case, the oxidation inhibiting agent preferably contains at least one kind of acid selected from the group consisting of phosphate, silicate, hypophosphite and tetrahydro borate. The adding amount of the oxidation inhibiting agent is preferably 1.0xc3x9710xe2x88x926 by mass % xcx9c5.0xc3x9710xe2x88x921 by mass % to the mass of the alloy. Further, if the oxygen exhausting step of vacuum-evacuating an atmosphere so that oxygen in the atmosphere is exhausted is done prior to the hydrogen gas reduction step, the surface of the hydrogen-absorbing alloy particle is prevented from being oxidized during the hydrogen gas reduction step.
Incidentally, the upper limit of the temperature range in the hydrogen gas atmosphere is preferably the temperature where the texture of the hydrogen-absorbing alloy is not subjected to the diffusion effect by heat. This is because if the texture of the hydrogen-absorbing alloy is disordered under the influence of heat, the activity of the hydrogen-absorbing alloy itself is attenuated. The temperature of the hydrogen gas atmosphere, which varies according to the kind of the hydrogen-absorbing alloy, about 100xc2x0 C.xcx9c500xc2x0 C. in order to treat the MmNi3.1Co0.9Al0.4Mn0.6. which will be described later
Further in order to attain the above object, method of manufacturing a hydrogen-absorbing alloy electrode according to this invention comprises:
a step of holding a hydrogen-absorbing alloy particle in an atmosphere of a hydrogen gas maintained at a temperature where absorbing of the hydrogen gas does not substantially occur;
a step of vacuum-evacuating the atmosphere of the hydrogen gas to exhaust the hydrogen gas; and
an adosorption step of cooling the atmosphere with the hydrogen gas exhausted and thereafter introducing an adsorption gas into the atmosphere so that the adsorption gas is adsorbed on the surface of the hydrogen-absorbing alloy particle.
Preferably, said step of holding comprises a step of reducing an oxide or hydroxide residing on the surface of a hydrogen-absorbing alloy particle while the alloy particle is held in an atmosphere of a hydrogen gas maintained at a temperature where absorbing of the hydrogen gas does not substantially occur. Meanwhile, if the temperature of the atmosphere is lowered during exhaustion of the hydrogen gas by evacuation of the atmosphere of the hydrogen gas, the hydrogen-absorbing alloy will adsorb the hydrogen gas. Therefore, in a state maintained at the temperature where absorbing of the hydrogen gas does not occur, the atmosphere of the hydrogen gas must be evacuated to exhaust the hydrogen gas. Even when the atmosphere is cooled to room temperature, since the hydrogen does not remain in the atmosphere, the hydrogen-absorbing alloy will not charge the hydrogen gas. When the adsorption gas such as carbon dioxide or nitrogen is introduced into the atmosphere in the state at a normal temperature (about 25xc2x0 C.), it will be adsorbed on the surface of the hydrogen-absorbing alloy particle.
Since the adsorption gas such as carbon dioxide or nitrogen adsorbed on the hydrogen-absorbing alloy particle is removed by the alkaline electrolyte in the battery, it does not exert an adverse effect on the battery. Since the active surface is exposed within the battery, the initial discharging capacity of the battery is improved greatly. Further, since the state where the oxide or hydroxide residing on the particle surface of the hydrogen-absorbing alloy has been reduced into the metallic state is maintained, the oxygen density within the hydrogen-absorbing alloy immediately after the battery has been manufactured decreases. Thus, corrosion/oxidation by the electrolyte within the battery and the oxidation by the oxygen generated from the positive electrode are retarded, thereby also improving the cycle characteristic.
Further, the method of manufacturing a hydrogen-absorbing alloy electrode according to this invention, comprises:
a hydrogen gas reduction step of reducing an oxide or hydroxide residing on the surface of a hydrogen-absorbing alloy particle while the alloy particle is held in an atmosphere of a hydrogen gas maintained at a temperature where absorbing of the hydrogen gas does not substantially occur;
a step of vacuum-evacuating the atmosphere of the hydrogen gas to exhaust the hydrogen gas; and
a step of cooling the atmosphere with the hydrogen gas exhausted to room temperature and thereafter introducing an argon gas or nitrogen gas into the atmosphere so that the pressure of the atmosphere is returned to normal pressure; and
a step of immersing the hydrogen-absorbing alloy particle thus acquired in a solution containing an oxidation inhibiting agent.
In this way, the hydrogen gas reduction treatment is performed to activate the surface of the hydrogen; the atmosphere of the hydrogen gas maintained at the temperature where absorbing of the hydrogen gas does not occur is vacuum-evacuated, thereby exhausting the argon gas; the argon gas or nitrogen gas is introduced to restore the pressure of the atmosphere to normal pressure; and the atmosphere is cooled to room temperature and thereafter the hydrogen-absorbing alloy particle is immersed in the solution. Thus, the oxidation inhibiting agent is selectively adsorbed on the surface of the hydrogen-absorbing alloy particle so that the creation of the oxide or hydroxide can be suppressed to maintain the activity.
In this case, the oxidation inhibiting agent preferably contains at least one kind of acid selected from the group consisting of phosphate, silicate, hypophosphite and tetrahydro borate. Where the hydrogen-absorbing alloy particle is immersed in the oxidation inhibiting agent, the alloy particle has preferably a smaller average particle diameter. This is because the alloy particle has a larger surface area as the average particle diameter decreases, and hence oxidized at a greater degree of oxidation. Further, if the oxidation/exhaustion step in which the atmosphere is vacuum-evacuated to exhaust the oxygen in the atmosphere is executed prior to the hydrogen gas reduction treatment step, it is possible to prevent the surface of the hydrogen-absorbing alloy particle from being oxidized during the hydrogen reduction treatment.
The temperature of the atmosphere of the hydrogen gas is set at a temperature not lower than the temperature where the hydrogen-absorbing alloy does not substantially charge the hydrogen gas. This is because when the hydrogen-absorbing alloy is held in the atmosphere of the hydrogen gas at a lower temperature than this temperature, the hydrogen gas does not contribute to the reduction of the oxide or hydroxide on the surface of the hydrogen-absorbing alloy particle, but is internally absorbed into the hydrogen-absorbing alloy particle from its surface.
When the temperature of the atmosphere of the hydrogen gas rises, the equilibrium hydrogen pressure of the hydrogen-absorbing alloy also rises so that hydrogen absorption of the hydrogen-absorbing alloy does not occur. The hydrogen gas in the atmosphere acts on the layer of the oxide residing on the particle surface of the hydrogen-absorbing alloy so that the nickel oxide and others on the surface of the hydrogen-absorbing alloy particle is reduced into the metallic state of nickel.
In order to prevent an impurity gas from being mixed into the hydrogen atmosphere from the treatment atmosphere, the pressure of the hydrogen gas atmosphere must be higher than the atmospheric pressure. Generally, it is preferably 1 Mpa (about 10 atm) exceeding the atmospheric pressure. It is more preferably 0.11xcx9c0.51 MPa (about 1.1xcx9c5 atm). The hydrogen-absorbing alloy is held for a sufficiently long time enough to reduce the layer of the nickel oxide or nickel hydroxide on the hydrogen-absorbing alloy completely. Concretely, it is preferably 0.5 hour or longer although it changes according to the treated amount of the hydrogen-absorbing alloy particle and other conditions.